Temp gauge reading at rest

[MilesA]

I am gradually working through the usual list of suspects in order to keep the 3A running at an acceptable temperature.

 

Yesterday I noticed that the temp gauge was registering a little north of 30 degrees. This on a car that has not moved for 3 weeks and was sitting in a cool garage.

 

Just wondered if this is the normal resting position of the needle or whether this might be indicative of an over-reading gauge. If the latter, it would explain a couple of things.

 

Anyone any knowledge?

 

As a supplemental, what are the pair of dots either side of the 85 degree line? Never noticed them before.

 

Many thanks as ever - Miles

 

 

~original

Edited February 7, 2018 by MilesA

[Ransomes256]

Miles,

I would consider the gauge to be inaccurate as the needle should be much lower. It would most be likely to read high throughout the range. The 2 pair of dots are calibration marks where a given resistance should put the needle in each of the zones.

Sorry I do not know what the calibration values would be.

Neil

 

[Guest ntc]

Miles

Not a tr3 gauge

[rhino_mac]

Agree. It's in degrees Celsius. TR3 is in Fahrenheit with 185 at 6 o'clock.

[RobH]

Miles - as the others have pointed out, the TR3A would have had a capillary type gauge not an electrical one. This must be an alteration using a gauge from a later mark and that raises a couple of questions. (By the way it is not clear from your post whether this is the reading cold but with the ignition turned on, or just un-powered).

 

1/ Does the car have an instrument voltage-stabiliser fitted? If not and the gauge is the type requiring a stabilised supply, it will read incorrectly anyway as it will be running from 12v instead of from 9-10v. The stabiliser was not fitted to the 3A as standard.

2/ There are several different temperature sensors for electrical gauges, with different resistance characteristics, and it is necessary to have the correct type else the reading will not be right.

[Lebro]

Miles.

Agree with Rob above, next time you visit the car give me a ring, & I will be happy to pop over & have a look.

 

Bob.

[j-eichert]

Excuse me for beeing from the continent, but there seems to be a "° Celsius"-version of the capillary instrument: This should be marked as TL2561/03.

 

So this could as well be a normal capillary instrument - if this is reading "high", there could be several reasons which can't be solved "at home".

It could be a bent mechanism inside, the C-spring itself may be bent or something is wring with the measuring bulb in the thermostat housing (reducing its volume -> resulting in higher reading)

 

Regards, Johannes

[RobH]

You are right of course Johannes, but the little calibration dots on the dial were only present on electrical gauges as far as I am aware, so I assume thats what it is.

 

By the way- I know we are an insular lot but I don't think you need to be excused for being from the continent !

 

(Some interesting trivia ---- when I was at school and college we used to call it Centigrade - Celcius did invent a similar range scale but his one ran backwards with 100C at freezing point and 0C at boiling. Centigrade was re-named in 1948 by the Conference General de Pois et Measures who decided that all temperature measuring scales should be named after a scientist- so they called it Celcius since his scale had degrees of the same magnitude. The new name took some decades to be widely adopted in the UK and there are still plenty of us who habitually refer to it as Centigrade. )

[j-eichert]

Thanks Rob

 

The additional dots are indeed puzzling, but if you look at the markings on the instrument, then you see:

- TL... for (T)emperature (L)ine = capillary instrument (mechanical)

- PL... for (P)ressure (Line) = mechanical pressure instrument

- FG/ PG... for (F)uel / (P)etrol (G)auge, direct electrical instrument

- BT... for (B)imetal (T)temperature = indirect electrical instrument

- BF... for (B)imetal (F)uel = indirect electrical instrument

 

So from this interpretation this instrument should be of the capillary type. A look at the wiring/ piping will clarify this for sure.

The markings could be around 50°C and 95..100°C (boiling water?)

In any case a calibration is necessary, so the dots could be used for calibration of the capillary as well.

 

Regards, Johannes

 

 

(Miles, if you don't like your wonderful shiny gauge: I take it and give you my worn, older with a °F-scale instead )

[MilesA]

Thanks all.

 

To respond.

 

Neil - Should have known it would not be the correct gauge although I had thought that both fahrenheit and celsius versions were available for the 3A.

 

Rob - The reading is shown when the car ignition was off.

 

I have previously wondered if there was a voltage stabiliser as I previously had the wrong type of fuel gauge and sensor (now changed to original) but I have never found one in the car.

 

Bob - thanks - I will be in touch.

 

Johannes - thanks for the offer - I will have a think about what I am going to do.

 

Actually, it would be great if this wrong gauge is over reading because it may mean that I do not have an over heating problem after all!

 

Whilst I do think about what to do (and extracting the bulb from the thermo housing is another potential can of worms), would fitting a voltage stabiliser and wiring the temp gauge to it be a simple short term measure that at least would give me an accurate temperature reading?

 

Miles

Edited June 20, 2017 by MilesA

[john.r.davies]

I was sure that TRaction had an article about rebuilding capillary temperature gauges, and that I had kept it, but I couldn't find it.

But no probs! The new TRaction Online has rescued me!

 

Miles, you're a member. Go to the Home page of this website, click on "Members Area" and login (bottom of the page) . Click on "TRaction Magazine Online" and search for "capillary". The article is first on the list, published in 2000.

John

 

PS I was delighted to read Wayne's editorial in this month's TRaction, announcing this service. I didn't think I'd need to use it so soon! Well done, TRR, for forward, modern thinking, about what the Club and website can do for members!

Edited June 20, 2017 by john.r.davies

[irrational]

Hi Miles.

 

I am no expert at all, but wouldn't it work to place the bulb in just-boiled water and see if the gauge reads at or just under 100 degrees?

I'd start by placing the bulb into cool/cold water and boiling that to gradually heat the bulb, then put it into a different boiling pot to measure.

I think read errors can occur if the connection at the gauge itself is bent or the mechanism inside. This is all book-knowledge though.

 

Dirk

[MilesA]

John - thanks - that is a great facility. From recollection I do not have a bulb screwed into the thermo housing, just a length of crimped metal about an 1" long (perhaps this is the correct sensor for the 'wrong' gauge that I appear to have). I will of course double check when disassembling the housing.

 

Dirk - yes, a simpler solution to testing the accuracy of the gauge! I can pour hot water into the upturned bowl of the housing and measure it with my food thermometer against the gauge.

 

These cars are never boring!

 

Miles

[RobH]

Confusion reigns I feel. Miles, if you have a capillary gauge there will be what looks like a thin metal rod with a spring wrapped around it, running from the bulkhead to the thermostat housing and usually clipped along the fuel pipe for support I and others were referring to an electrical gauge which just has an electrical wire to (usually) a spade connector on the rear of the sender - its that type which can have the wrong sensor and which needs a voltage stabiliser.

 

The capillary gauge is all-in-one and the sensor cannot be disconnected from the gauge. The sensor end is a metal 'bulb' full of liquid, which goes right into the thermostat housing and is held in with a threaded gland nut. If it is one of those, beware of trying to remove the sensor bulb if it has been in place for any length of time. The sender and particularly the capillary tube are easy to damage in trying to unscrew the nut and remove the bulb from the housing - the whole thing can turn with the nut, as they may be corroded together and that will twist the pipe. If it fractures the diethyl ether will escape and the gauge will then definitely need re-gassing. The same thing may happen if you remove the thermostat housing with the sensor in situ unless you are very careful not to twist the pipe sharply in the process.

 

if you just want to check the calibration the easiest thing to do if you have access to an infra-red thermometer is to run the engine up to normal temperature and check the housing temperature with that.

[Don H.]

IR thermometers can be quite useful, but beware of errors than can result from bright shiny surfaces. (Check emissivity for lots more detail).

 

A relatively simple way to improve accuracy is to put black paint or a piece of black electrical tape on the surface being measured. (Assuming it's not too hot for the tape, of course.)

[irrational]

I second the IR thermometer if you have one.

I have one that I bought to play with.

It is accurate enough. Within 3-5 degrees I'd say and mine is a cheapie.

I have used it on the housing and it seemed ok, but I just wanted a ball park figure. I wasn't sure if it is accurate enough to trust fully.

[MilesA]

Rob

 

Confusion reigns upon confusion which of course is not uncommon with 50 year old cars that have been adapted by POs along the way.

 

I do have the thin metal rod, protected by a spring running round it running from the thermo housing to the bulkhead clipped to the fuel pipe for support which is why I assumed I had the correct capillary temp gauge.

 

The two oddities now seem to be - an incorrect gauge face and something other than a bulb projecting into the thermo housing.

 

With regards to the bulb I am away from the car and do not have a photo of what is actually in the housing. I am sure there is something other than a bulb but cannot examine for a few weeks. So apologies, I will have to clarify this later.

 

So far as the gauge face is concerned, if I have a 3A capillary system, it may be that a PO has replaced the original Fahrenheit face with a later Celsius one. Maybe all I need is a face change. Would be interesting to know if the 90degree position on the face is at the same point as the 30 degree point. Can the capillary gauges over-read I wonder.

 

Miles

[Lebro]

If the gauge does go up & down with changing temperature, then it is not broken, I does seem that you have the correct capilliary type, but with °C scale.

It would almost certainly be possible to re-calibrate it to be accurate (well more accurate anyway). I have done this on mine.

 

Bob.

[MilesA]

Bob - looks like confusion is receding and a plan is emerging. In will catch up some time in my usual vague way. In the meantime I will see if I can hunt down the correct face.

 

Thanks all - Miles